1. Field of the Invention
The present invention relates in general to a catalytic converter for use in a motor vehicle to purify the gases exhausted from an engine of the vehicle, and more particularly to a method of producing the catalytic converter. More specifically, the present invention is concerned with a method of producing a metal support for exhaust gas catalysts, which support is an essential element of the converter.
2. Description of the Prior Art
In order to clarify the object of the present invention, one conventional method of producing a metal support for exhaust gas catalysts will be described with reference to FIGS. 4A, 4B, 4C and 4D of the accompanying drawings. The method is disclosed in Japanese Utility Model First Provisional Publication 63-54432.
In this method, as is shown in FIG. 4A, at first, a flat metal strip 22 and a corrugated metal strip 23 are mated at their leading ends and, as is shown in FIG. 4B, they are wound around a common (cylindrical holding member 21) by a predetermined number of times to provide a cylindrical structure 24. Then, as is shown in FIG. 4C, the (cylindrical holding member 21) is drawn out at from the cylindrical structure 24. Thus, a semi-processed cylindrical assembly 24 with a center bore 25 is produced. Then, as is shown in FIG. 4D, the cylindrical assembly 24 is pressed in such a manner that the center bore 25 becomes flatly deformed. With this, the assembly 24 assumes an oval shape, which is generally designated by numeral 26. A suitable welding is then applied to the oval assembly 26 in order to keep the oval shape of the same. Then, by using a known catalyst coating technique, a suitable catalyst is coated on inner surfaces of the honeycomb cells thus defined by the oval assembly 26. Then, the fixed and catalyst-coated oval assembly 26 is housed in a shell (not shown) to provide a catalytic converter.
However, the method has the following inherent drawbacks to its inherency.
That is, as will be understood from FIGS. 4C and 4D, when the semi-processed cylindrical assembly 24 is pressed to assume the oval assembly 26, so-called "pockets" 27a and 27b at lateral ends of the flatly deformed bore 25 are inevitably formed. This undesired phenomenon will be better understood from FIG. 5 in which the phenomenon is shown exaggerately.
As can be seen from FIG. 5, upon completion of the pressing process the upper and lower inner walls 26a and 26b of the flatly deformed bore 25 get in contact with each other at their respective middle portions. However, at the laterally opposed ends 26c and 26d of the flatly formed bore 25 where a great change of curvature occurs during the pressing process, such contact does not occur causing the formation of the pockets 27a and 27b. Thus, if the catalytic converter is used with such an oval assembly 26 contained therein, the pockets 27a and 27b cause considerable decrease of the gas purification which is carried out by the catalytic converter. In fact, the exhaust gas from the engine is permitted to pass through the pockets 27a and 27b without being purified. Of course, such gas passing can be prevented by closing the pockets 27a and 27b with the catalyst in the catalyst coating process. However, in this case, a greater amount of catalyst is needed and thus the production cost of the converter is increased considerably.